Grass

S1, E3: The Great Rift Valley provides the stage for an epic battle between trees and grass, its course influenced by volcanic eruptions, landscape, and rainfall. On its outcome rests the fate of Africa's great game herds. In the Rift's savannas, grazers and their predators struggle to outwit each other, forcing one group of primates to develop a social system that paved the way for the evolution of mankind.